Engine Science Camshafts and Pistons

A camshaft primer: the mystique of valve motion … as regulated by the shaft with the cams on it

To many of us, knowledge of an engine’s camshaft stops at lift and duration. Or perhaps it doesn’t even really go that far. So often, it seems, we tend to understand and select a particular cam on its merits as told to us by someone else, perhaps one whose engine combination is especially applicable to the cam he’s using, but not necessarily best suited to our specific engine.

The subject of cam function and design is a rather large one, so it is the intent of this month’s Series to deal with the ground-level basics of cams: What they do. How they do it. What terms are used to describe their parts. And what these terms mean. So that by the time you get around to picking your next cam, there’ll be a little more substance to your selection than, “Gimme that one ’cause I like its looks.” We know because we’ve been there. First of all, suppose we discuss what a camshaft is supposed to do in an internal combustion engine.
Air and fuel pass into an engine

and spent exhaust gas leaves it. And since the combustion process deals with very high cylinder pressures, intake and exhaust valves must seal and hold this pressure to provide usable power. Other than during these times, intake and exhaust valves must be opened and closed to allow the passage of air/fuel mixtures and exhaust gas into and out of the engine’s cylinders. Couldn’t get much simpler, right? But it is the timing of these openings and closings and the duration of these events that govern how a given camshaft affects power output.
On a common shaft there are typically located several noncircular lobes. As the shaft turns (make a good soap opera title, wouldn’t it?), each of these lobes can impart motion to a follower (lifter) that causes a rocker arm to open and close a particular valve. In pushrod engine design, there’s no direct contact between follower and rocker arm (a pushrod connects them). In overhead cam engine design, camshaft lobes impart direct motion to rocker arms.

But regardless of the specific method of valve operation, camshafts with lobes cause valves to (1) open, (2) close, (3) remain open, (4) be lifted a specific amount from the valve seat. Obviously, it is the design (shape) of each lobe and its position relative to other lobes on the shaft that affect valve motion and the effects of valve timing on engine performance.
Perhaps the most important aspect of camshaft design, selection and understanding is where in an engine’s rpm range are optimum volumetric efficiencies going to be required. Cams designed for low-rpm engine operation are not the same as those intended for higher rpm use. Discussion of differences among these types will follow. But to get us into that, let’s now spend a few minutes talking about some of the terminology relative to cam design.
Base circle is not the ring drawn on the ground to locate home plate. It’s the lobe circle (as shown in the illustrations) from which additional radiuses are referenced to cause valve motion.

A. Here you can see the relationships of base circle, lobe lift and valve duration. During times when a valve is in its closed position, follower (or lifter) travel is along the base circle surface of the lobe. From the time when a valve leaves its seat to when it is seated again, “off-seat” time (or valve duration) allows mixture/gas flow. Note that net lobe lift is measured from base circle radius to optimum lift, not from the point at which lift motion begins. B. The angular measurement (in crankshaft degrees) from the centerline of an intake valve lobe to the centerline of a corresponding exhaust valve lobe is called “lobe displacement angle.” Variations of this relationship govern the amount of “overlap” between intake and exhaust valves.

This entry was posted on Thursday, April 15th, 2010 at 4:41 PM and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.